What the radiograph says and what it does not
- A fractured instrument in the canal is clearly visible on a radiograph, but on its own it does not settle the fate of the tooth. What decides the prognosis is whether the canal had been cleaned before the fracture happened.
- An underfilled canal, that is a filling that ends too high, short of the root apex, is associated with more frequent inflammation at the root apex, though not in every patient.
- A perforation heals well when it is sealed tightly, before infection has had time to settle in it. The fact that a tooth healed within a year does not close the matter, however: recurrences come after years.
- Material extruded beyond the apex usually requires nothing. Two exceptions are urgent: numbness of the lip, the chin or the tongue, and material that has ended up in the maxillary sinus or in the mandibular canal. Increasing pain or swelling is also a reason to contact the practice.
What can actually be seen on a radiograph after root canal treatment?
The radiograph shows three layers of information: the dense band of material in the canal, the outline of the root, and what is happening in the bone around the apex. It does not show bacteria, it does not show the seal, and it does not show pain. It also shows less than one might think. Experienced endodontists looking at an ordinary periapical radiograph found a little under two thirds of the lesions that tomography showed in the same tooth. This was counted on 339 teeth referred for first-time root canal treatment; 121 of them had a lesion. In the other direction they were wrong rarely: among 218 teeth in which tomography showed no lesion at all, they thought they could see one on the radiograph in only five.
The figures were measured almost exclusively on molars and exclusively on teeth not yet root canal treated. In a tooth already filled, tomography too is wrong more often - that is what emerges from a review of studies in which the image was compared with examination of the tissues under a microscope, on cadaver material. The practical rule stays the same, however: a shadow at the apex visible on a radiograph is most often real, and its absence settles little.
An underfilled canal: when is it a problem?
When the empty stretch of the canal is infected. The gap on its own, between the end of the filling and the root apex, is a technical measurement. You will not build a diagnosis on it: it tells you how many millimetres are missing, and there it ends. In the Oslo study from which the figures below come, a filling ending on the radiograph more than three millimetres from the apex was counted as too short. Among teeth in which the filling ended too high, 53 per cent had inflammation at the apex. The background figure is 35 per cent among all root filled teeth in the same sample. The figures come from a single cross-sectional examination of 450 randomly selected sixty-five-year-olds from Oslo; there were 756 root filled teeth among them.
Two reservations change how those figures are to be read. This is a cross-sectional study, that is a picture of a single moment, so it does not distinguish a healing lesion from a growing one. Nor is it known what diagnosis the tooth carried when it entered treatment, and a filling reaching the apex means one thing in a tooth that was healthy before the procedure and another in an infected one.
An underfilled canal raises the risk but does not settle it.
A little under half of the teeth with a filling of that kind had no diagnosed inflammation at the apex in the Oslo sample.
Three questions bring order to the assessment of such a radiograph:
- Is the lesion in the bone at the apex larger than on the previous radiograph?
- Does the tooth give symptoms: pain on biting, tenderness, a sinus tract?
- Is the restoration above the canal sealed?
A fractured instrument in the canal: remove it or leave it?
Not always remove it. What decides is the moment at which the instrument fractured.
The fractured instrument itself is usually brighter on the radiograph and more sharply demarcated than the canal filling, and its course is sometimes different from the course of the material surrounding it.
A little over two fractures per hundred instruments used - that was the pooled frequency in a review of clinical studies of rotary nickel-titanium instruments. It was counted on close to 33 thousand instruments. An expert consensus from 2025 gives a wider range for all types of instrument: from a quarter of one per cent to ten per cent. This is an agreement between specialists, so it carries less evidential weight than a study with random allocation. In the same review, fractures were higher among less experienced operators. The risk also rises in curved and calcified canals.
The same consensus puts the matter this way: in a canal without infection, or in one that had been cleaned in time, leaving the fragment in place does not necessarily increase the risk of failure. The prognosis is worse for an instrument fractured in an infected canal that had only just begun to be prepared, because the fragment then blocks access to the space that had not yet been disinfected.
On the other side stands the price of removing the instrument. The chance of retrieving the fragment falls from 83 to 43 per cent once the canal curves by more than twenty degrees, and you will not judge that by eye: the angle is measured by the dentist on the radiograph, before any attempt is made to retrieve the fragment. Most nickel-titanium fragments are about three millimetres long and sit in the apical third of a molar canal, most often at the curve itself, that is where retrieval is hardest. To remove a fractured instrument, dentine has to be taken away around it, and the deeper in the root the fractured instrument lies, the more dentine is lost and the weaker the root becomes. According to the same consensus, removing a fragment from the coronal third of the canal does not worsen the resistance of the tooth to fracture. With fragments lying deeper, the same procedure begins to put the tooth at risk.
The fragment is retrieved by the dentist under magnification.
What a microscope really changes we described separately.
In conversations with patients presenting at Modern Dental & Orthodontics (Klinika MDO), the question of whether metal left in the root harms the body comes back repeatedly - and that question is put in the wrong place. A fractured instrument in the canal counts above all as an obstacle to cleaning it. It is not a foreign body that has to be removed for the sake of general health.
Perforation of the root wall or of the pulp chamber floor: what decides the prognosis?
It depends on how tightly the perforation was sealed and after how long. A tooth perforation is an unnatural connection between the inside of the tooth and the tissues around the root; in root canal treatment it arises as a complication of the procedure. It opens either in the wall of the root or in the floor of the pulp chamber, that is in the floor of the space from which the canals run; the latter lies closer to the division of the roots and falls on the radiograph in a different place than along the outline of the root. The perforation itself is usually not visible on the radiograph - what is visible is its consequence: loss of bone at the lateral wall of the root, or material lying outside its outline halfway along the root, away from the apex. The outcome deteriorates above all when bacteria have had time to settle in that place; this is what a review devoted to the management of perforations indicates.
A sealed perforation heals in nine teeth out of ten, and quickly at that. At the first annual follow-up, 110 of 124 teeth were healed, one tooth per patient. That is what the early result of a fourteen-year observation says, in which perforations were sealed with a calcium silicate-based material. The continuation of that work warns against stopping at that result. Among the teeth counted as healed, the estimated probability of recurrence came to 6 per cent after five years, 30 per cent after ten and 62 per cent after fourteen. Recurrence means one thing in that paper: a tooth previously counted as healed returned to the unhealed group, under the same clinical and radiographic criteria. It does not mean that the tooth was extracted; the publication does not say what was done with those teeth afterwards.
Faring worse in the process were perforations larger than three millimetres, and those alongside which a probe entered a pocket running along the root, that is those that were still connected with the gingival margin. This is a single cohort from one centre, so the figures should not be transferred to your own tooth. Those two features do, however, move an individual patient up or down relative to the cohort, and both are assessed by the dentist during examination in the surgery, not on the radiograph. A tooth with a repaired perforation needs follow-up for years, not for a year.
Material extruded beyond the apex: what does it mean?
Most often nothing that could be felt. Extrusion of material beyond the apex is visible on the radiograph as a fine streak of sealer outside the outline of the root. Teeth with extruded sealer fared worse under the loose criterion and the difference barely crossed the threshold of statistical significance. That study compared two ways of filling the canal and was not designed to assess extrusion. It covered 74 teeth followed for seventeen months on average. It did not track the fate of the streak itself on subsequent radiographs.
The situation looks entirely different when material or an instrument ends up in the mandibular canal or in the maxillary sinus. Extrusion of material and preparation of the canal beyond the apex accounted for a quarter of trigeminal nerve injuries after endodontic procedures. That figure was gathered from 47 cases referred to a single facial pain clinic over seventeen years. Patients whose sensation improved reached the specialist after a little under nine weeks on average; those without improvement waited more than forty weeks on average. The paper is a review of records, so it does not prove that it was speed alone that brought the improvement - what it does show is which side the risk of delay falls on.
Numbness of the lip, the chin or the tongue that persists after the anaesthetic has worn off is to be reported the same day. Do not wait for the follow-up appointment.
Which complication needs urgent treatment and which only follow-up
| What is visible on the radiograph | Default flag | What moves the flag upwards |
|---|---|---|
| A fractured instrument in the canal, a tooth without complaints, no lesion at the apex | follow-up | appearance of a lesion, pain on biting |
| A filling ending too high, no lesion | follow-up | a lesion in the bone, a sinus tract, a leaking restoration |
| A filling ending too high together with a lesion at the apex | planned treatment | growth of the lesion between radiographs |
| A fresh perforation, of which the dentist informed you at the appointment | urgent treatment | the highest flag; sealing at that appointment |
| An older perforation with loss of bone | planned treatment | deep probing, mobility of the tooth |
| Material beyond the apex, without symptoms | follow-up | pain, numbness, swelling |
| Material or an instrument in the mandibular canal or in the sinus; numbness after treatment | urgent treatment | sensory disturbance persisting after the anaesthetic has worn off; contact the same day |
The labels alone say nothing about time, so it is worth translating them into a calendar: sensory disturbance and spreading swelling are a matter for today, a fresh perforation for this appointment, a lesion at the apex for the coming weeks, and an unusual picture without symptoms for a scheduled follow-up visit.
The label planned treatment does not settle what that treatment is. When root canal retreatment makes sense, and when a surgical procedure is the better route, is the subject of a separate text. The flag is assigned by the dentist once the radiograph has been set alongside the examination in the surgery - that is, within what an endodontic consultation covers.
Standing apart from the flag is the restoration above the canal: a leaking crown or a leaking filling lets bacteria back into the cleaned canal, so restoring a tooth after root canal treatment is part of the prognosis.
The follow-up flag does not mean forget about it.
It means: come for an examination and a radiograph at the agreed time, usually after a year, so that two images of the same place can be set side by side. Both radiographs also have to be taken with the tube positioned in a similar way, because otherwise you are comparing not a change in the bone but two different projections of the same root. This is worth asking for when handing over the previous radiograph for review.
Standing apart are the symptoms that need no radiograph for a decision to be taken: spreading facial swelling with fever, difficulty opening the mouth or swallowing. These are indications for urgent help regardless of what is visible on the film.
When is tomography needed?
When the answer will change what the dentist does next. With a fractured instrument in the canal, the expert consensus recommends both a periapical radiograph and tomography before any attempt at retrieving the fragment. Tomography shows the thickness of the dentine, which is not visible on an ordinary radiograph, while the periapical radiograph renders the material of the instrument itself better, because in tomography metal produces artefacts.
For assessing the outcome of treatment itself, on the other hand, tomography is sometimes unnecessary. Under the loose criterion, the result from tomography does not differ clearly from what an ordinary radiograph gives. This is pointed out by the authors of the pooling of 19 studies in which healing was assessed by tomography.
The same tooth can be healed or unhealed depending on which criterion is applied.
So when someone says that root canal treatment has a success rate of a given percentage, the first question is: counted by which criterion.
Frequently asked questions
Does a fractured instrument always have to be removed?
No. The expert consensus indicates that a fractured instrument in a canal without infection, or in a canal cleaned before the fracture, does not necessarily increase the risk of failure. The decision is weighed together with the cost of retrieval, because removing dentine around a deeply lying fragment weakens the root. The choice depends on the position of the fragment, the curvature of the canal and the state of the tissues at the apex.
Does an underfilled canal always require retreatment?
No. In the study on a random sample of 65-year-olds from Oslo, 53 per cent of teeth with a filling ending too high had inflammation at the apex, which means that the remainder did not. What decides the indication is the lesion in the bone, the symptoms and the seal of the restoration above the canal. The measurement of the distance from the apex does not settle it on its own.
Can a perforation be repaired?
Yes. In the cohort of 124 teeth sealed with a calcium silicate-based material, 89 per cent were healed after a year. The same work shows, however, that some of the teeth counted as healed return in later years to the unhealed group. A perforation larger than three millimetres had a worse prognosis. A repaired tooth needs follow-up over many years.
Does a complication always mean the dentist made a mistake?
No. Instrument fracture happens to experienced operators too, and the frequency rises in curved and calcified canals. Badly done root canal treatment is a conclusion that is not drawn from a radiograph: the image shows the consequence, not the course of the procedure. The answer comes only from the record of how the treatment proceeded. We do not assess the work of other practices.
The tooth hurts after root canal treatment and nothing is visible on the radiograph. What does that mean?
That the radiograph does not rule it out. In the study on 339 teeth, experienced endodontists recognised on an ordinary radiograph a little under two thirds of the lesions that tomography showed. Persisting pain is therefore an indication for examination in the surgery and for considering tomography. Waiting until the lesion becomes visible is not a strategy here.
What should be in the records I can ask for?
Radiographs taken before treatment, during it and after it, a description of the course of the appointments with dates, information about the complication identified and the management undertaken, and follow-up recommendations. A patient has the right to apply for a copy of the records at the practice where they were treated. This set allows the next dentist to establish when the event occurred and whether the canal had been cleaned before it.
What the radiograph will not settle for you
The radiographic image answers one question best: whether something can be seen in the bone at the apex that was not there on the previous radiograph. A fractured instrument in the canal, underfilling and a streak of material outside the root are a description of a state, not a diagnosis. Everything beyond that is a conclusion drawn from setting the image alongside the examination and the treatment history.
What is at stake is the tooth. The longer a complication goes unrecognised, the shorter the list of possibilities becomes.
The hierarchy of urgency is simple, and it is worth remembering in this order: sensory disturbance and spreading swelling are a matter for today, a fresh perforation is a matter for this appointment, a lesion at the apex is a matter for the coming weeks, and the mere sight of an unusual picture without symptoms is a matter for a scheduled follow-up visit.
Read more:
- Root Canal Treatment in Warsaw
- Root canal treatment under a microscope — what magnification really changes
- Dental radiographs, panoramic imaging, 3D tomography: A guide to dental radiology
- A crown on a tooth after root canal treatment - when it is necessary
- Root canal retreatment after failed treatment — when it makes sense and when it does not
Content and liability disclaimer
This article is informational and educational in nature and does not constitute medical advice, a diagnosis or a therapeutic recommendation — it does not replace a consultation with a specialist. If you are experiencing symptoms, have doubts or are facing a decision about treatment, consult a dentist. The methods described and the data cited are general in nature. The results of scientific studies relate to populations, not to an individual patient. The course and outcome of therapy depend on the individual clinical situation and may vary between patients. No information contained in this article constitutes a guarantee of result. The content has been prepared with due care, based on publicly available medical knowledge and the scientific publications indicated in the Sources section. We do not advise taking or refraining from any health-related action solely on the basis of the content of this article, without prior consultation with a doctor.
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